As documented by Wikipedia, vocalist Robert Plant said in a bootleg recording from a 1970 show:
This is a thing called â€˜Bron-Yr-Aurâ€™. This is a name of the little cottage in the mountains of Snowdonia in Wales, and â€˜Bron-Yr-Aurâ€™ is the Welsh equivalent of the phrase â€˜Golden Breastâ€™. This is so because of its position every morning as the sun rises and itâ€™s a really remarkable place. And so after staying there for a while and deciding it was time to leave for various reasons, we couldnâ€™t really just leave it and forget about it. Youâ€™ve probably all been to a place like that, only we can tell you about it and you canâ€™t tell us.